1. Field of the Invention
The present invention relates to a method and portable apparatus which is used to detect substances, such as explosives and drugs, by neutron irradiation. More particularly, the present invention is directed towards an apparatus which utilizes gamma ray detection, said gamma rays emitted by the interrogated object after subjecting it to a neutron burst thereby providing the ability to calculate the concentration of an element in the object of interest.
2. Discussion of the Prior Art
High explosives and illicit drugs are primarily composed of the chemical elements of hydrogen, carbon, nitrogen and oxygen. Systems for the detection of these elements are fairly well known. These systems utilize the irradiation of such material by neutrons and detection of gamma photons emitted by the materials after subjecting them to said neutron burst. One technique of such detection is neutron activation analysis. These prior art devices utilize the known effect of gamma ray emission from the nucleus of the objects being interrogated after irradiation. The concentration of these gamma rays can be detected by gamma ray detectors and the signals analyzed to determine the concentrations of chemical elements which make up the object being interrogated. These elements are found in explosives or illicit drugs in differing quantities, ratios and concentrations. By calculating and determining said ratios and concentrations, it is possible to identify and differentiate drugs and other contraband from sugar or other materials by measuring the amount of hydrogen, carbon or other material contained in the interrogated object.
It is further understood that the process of producing gamma photons by the interaction of the nucleus of the inspected material with neutrons from a neutron generator can be effected by either of three processes. These include fast neutron, thermal neutron and neutron activation reactions. Thermal neutron reactions occur by the capture of a neutron by a nucleus producing an isotope which is de-excited by the emission of gamma radiation. Fast neutron reactions result in the inelastic scattering of a neutron on a nucleus which is de-excited by the emission of prompt gamma radiation, this interaction occurring only with fast neutrons having a high enough energy which is at least equal to that of the prompt gamma radiation. Finally, neutron activation reactions occur by the activation of a nucleus by a thermal or fast neutron which creates a radioactive nucleus having a certain life and which disintegrates thereby emitting activation gamma radiation.
Further, prior art devices which are disclosed in U.S. Pat. Nos. 5,293,414, 4,864,142 and 5,373,538 suffer from other problems including the non-portability of their devices and the effect of producing false positive results when interrogating the object. Such false positives can occur due to background radiation, failure to account for the orientation of the interrogated object and detectors and the statistical analysis which may cause a miscalculation of the occurrence of the appropriate materials. Most importantly, the need for an accurate and portable system for emission and detection of neutrons and gamma rays has been needed in order to properly allow the detection of explosives and other material in varying environmental situations and circumstances.
The present invention is for a portable method and apparatus for detection of particular elements by neutron irradiation. The apparatus utilizes a portable self-contained probe which contains gamma ray detectors and associated electronics, a neutron generator and power supply and shielding material which isolates the detectors from the neutron generator. The probe is small and is of such design that it may be placed in between or near interrogated objects. The probe additionally contains appropriate high voltage and low voltage power supplies in addition to associated data collection electronics. The probe is remotely connected to a low voltage power supply controller and a controller which properly maintains and controls the neutron generator. Further, the probe is remotely connected to a data acquisition computer which detects the appropriate signals generated by the gamma ray detectors and converts them to digital values. These values are then analyzed by elemental characterization software which determines in real time which constituent elements are present in the interrogated object.
Finally, the present invention comprises a portable pulsed neutron detection system for detection of specific elements in an object, comprising: a portable probe, said probe having a pulsed neutron generator and at least one gamma ray detector; a controller operably connected to said pulsed neutron generator for varying the intensity and pulse characteristics of said pulsed neutron generator so as to emit a beam of neutrons from said generator; a data acquisition system operably connected to said at least one gamma ray detector for collecting data measured by said detector; and, means to analyze said data to determine the chemical composition of said object.